A study has found that patients given a blood thinning drug to restore blood flow in the brain within 90 minutes of their symptoms were two and half times more likely to have a good recovery than those not given the drug.

Patients did still benefit from the drug, called alteplase, when given later but to a lesser extent, doctors said in The Lancet medical journal.

Currently recommendations are that the drug be given within three hours of the first symptoms appearing but it was not known whether patients who missed this window would still benefit.

An analysis of 3670 patients in eight studies showed that the earlier the drug was given the greater the chance the patient would recover without suffering lasting disabilities.

But even those who received it up to four and half hours after their symptoms started, still showed some benefit and were 22pc more likely to have a good recovery than those not given the drug.

The analysis found that patients treated within 90 minutes were 22pc less likely to die, but the chance of dying increased with the length of time it took for treatment to start.

Lead author Prof Kennedy Lees, from the University of Glasgow said: “Our analysis showed that the greatest benefit comes from earlier treatment, since net benefit is diminishing and is undetectable in our sample beyond 4.5 hours.

"However, alteplase does not result in excellent benefit in most patients, even those treated early, so questions remain.

“We need to understand better the factors that prevent alteplase from being effective in individual patients.

“Patients with ischemic stroke selected by clinical symptoms and CT benefit from intravenous alteplase when treated up to 4·5 hours. To increase benefit to a maximum, every effort should be taken to shorten delay in initiation of treatment. Beyond 4·5 hours, risk might outweigh benefit.”

In an accompanying comment article, Prof Jeffrey Saver of the Geffen School of Medicine in American and Prof Steven R Levine of Mount Sinai School of Medicine, said: “We need to increase the proportion of patients arriving at hospital in the first golden hour after ischemia onset by better educating the public to recognise stroke warning signs and by activating the emergency medical system early”.

They added that stroke centres should target “the improvement of hospital-response systems to achieve door-to-needle times of less than 60 min” in most alteplase-treated patients.

Dr Sharlin Ahmed, Research Liaison Officer for The Stroke Association said: “Previous research has shown that clot busting treatments are most effective if they are administered within three hours of the on-set of stroke symptoms. However, this study suggests that clot busting treatments can still be beneficial to some stroke patients after the three hour time window, up to four and half hours.

"After this point the risks of the treatment outweigh the potential minimal benefit to the patient.

"Unfortunately, not all stroke patients are eligible to receive clot busting drugs, however, for those who are, this research supports the idea that the faster the drug is administered, the greater the potential is for recovery.

"It is therefore fundamental that people view stroke as a medical emergency and get the patient to hospital as quickly as possible."